1. Field of the Invention
This invention relates to a transmission control apparatus for a duplex loop type transmission system, and more particularly to a transmission control apparatus which is capable of immediately clearing trouble occurring on transmission lines.
2. Description of the Prior Art
FIG. 1 is a schematic view of a duplex loop type transmission system disclosed in the Japanese Patent Laid-Open No. 57-162531 (1982), in which four transmission stations 3a, 3b, 3c and 3d are connected in a loop by means of transmission lines 1 and 2 through which signals flow reversely to each other, the four stations being adapted to sequentially have the control right for the entire system.
FIG. 2 is a block diagram showing constitution of the transmission station and the transmission stations 3a, 3b, 3c and 3d each have the same construction.
In the drawing, reference numeral 3 designates a data transmit-receive control circuit, 5 through 10 designate switching circuits for switching the transmission lines, 11 designates a switching control circuit for the switching circuits 5 through 10, 12a and 12b designate detection circuits of a network switching signal for a local station, 13a and 13b designate delivery circuits for a network switching command to perform the network switching control for other stations, and 14 designates a trouble control circuit which carries out trouble control by a trouble processing demand signal from the data transmit-receive control circuit 3.
Next, explanation will be give on operation of the conventional transmission control apparatus.
As shown in FIG. 1, it is assumed for explanation that the transmission lines 1 and 2 are disconnected at the points X between the transmission stations 3c and 3d, at which time the station 3a has the control right.
The data transmit-receive control circuit 3 at the transmission station 3a detects an occurrence of trouble on the transmission lines due to non-response to the data transmission or a frame error detection by the cyclic check, and then outputs a trouble processing demand 51 with respect to the trouble control circuit 14 at the station 3a, so that the trouble control circuit 14 carries out the loopback control and continuity test of the respective transmission stations 3b through 3d by use of one transmission line 1.
In detail, at first, the network switching command delivery circuit 13a delivers a loopback test command for the transmission line 1 to the transmission station 3b via a switching circuit 15a on the transmission line 1. Then, the transmission station 3b receives the command signal by the detection circuit 12a of the network switching signal for the station 3b and closes the switching circuits 5 and 8 through the switching control circuit 11, thereby making the transmission lines in the system switching condition for the loopback of the transmission line 1. In other words, the signal sent through the transmission line 1 is returned to the transmitting side from the transmission line 2 through the data transmit-receive control circuit 3.
Next, the trouble control circuit 14 at the transmission station 3a delivers a signal to the switching control circuit 11 to close the switching circuits 6 and 7 so as to make the system switching condition for the loopback of the transmission line 2, and a test data delivery signal 52 is delivered to the data transmit-receive control circuit 3 to thereby allow the circuit 3 to transmit the test data to the transmission station 3b.
Then, the test data returns to the station 3a via the switching circuit 5, data transmit-receive control circuit 3, switching circuit 8, and switching circuit 15b interposed in the transmission line 2 at the transmission station 3b.
The test data having thus returned to the transmission station 3a is received by the data transmit-receive control circuit 3 through the switching circuit 6. The data transmit-receive control circuit 3 feeds to the trouble control circuit 14 a reception confirming signal 53 indicating the reception of the test data. The trouble control circuit 14 receives the signal 53 to decide that the transmission lines between the transmission stations 3a and 3b are normal so as to deliver a straight command for the transmission line 1 to the transmission station 3b from the network switching command delivery circuit 13a through the switching circuit 15a. At the transmission station 3b, the command is received by the detection circuit 12a for the network switching signal for the station 3b and the switching circuits 5 and 7 are closed through the switching control circuit 11, thereby making the transmission line 1 switch into a straight condition. In other words, the signal transmitted through the transmission line 1 is returned thereto through the data transmit-receive control circuit 3.
Thus, the loopback control and continuity test for the transmission station 3b is completed and similarly the transmission station 3a carries out control for the transmission stations 3c and 3d. In this case, since the transmission lines are disconnected at the time when the transmission station 3d is subjected to the continuity test, the test data reception confirming signal 53 is not given to the trouble control circuit 14 at the transmission station 3a, whereby abnormality between the transmission stations 3c and 3d is checked by the trouble control circuit 14. Hence, the transmission station 3c is reswitched to the transmission line 1 system loopback, thereby completing the test and the network formation to the transmission line 1.
Next, the loopback control and continuity test of the line 2 for the transmission stations 3d through 3b are carried out from the transmission station 3a with respect to the transmission line 2 in the same way as the line 1. The transmission station 3d, one station before that 3c, where the abnormality has been detected, carries out the loopback of the transmission line 2, so that the portions at the transmission lines 1 and 2 between the transmission stations 3c and 3d are separated from the loop as shown in FIG. 3, thereby forming the closed loop at the respective transmission stations 3c and 3d, thus completing the trouble control.
In addition, in FIG. 2, reference numerals 35 through 40 designate control signals given from the switching control circuit 11 with respect to the switching circuits 5 through 10 respectively.
The conventional transmission control apparatus for duplex loop type transmission system constituted as foregoing, is controlled sequentially, thereby requiring much time to clear the trouble.